Pyrochlore electrodes

ABSTRACT

ELECTRODES USEFUL FOR ELECTROCHEMICAL REACTIONS ARE DISCLOSED. ALSO DISCLOSED ARE ELECTROLYTIC CELLS USING SUCH ELECTRODES AND THE USE OF SUCH ELECTRODES IN THE CONDUCT OF ELECTROCHEMICAL REACTIONS. THE ELECTRODES HAVE A PYROCHLORE-CONTAINING SURFACE ON A SUITABLE ELECTROCONDUCTIVE BASE.

"United States Patent Office Patented Apr. 2, 1974 3,801,490 PYROCHLOREELECTRODES Cletus N. Welch, Clinton, Ohio, assignor to PPG Industries,Inc., Pittsburgh, Pa. No Drawing. Filed July 18, 1972, Ser. No. 272,823Int. Cl. B01k 3/06; C01b 11/26 US. Cl. 204-290 F 9 Claims ABSTRACT OFTHE DISCLOSURE Electrodes useful for electrochemical reactions aredisclosed. Also disclosed are electrolytic cells using such electrodesand the use of such electrodes in the conduct of electrochemicalreactions. The electrodes have a pyrochlore-containing surface on asuitable electroconductive base.

BACKGROUND Numerous electrochemical reactions such as the electrolysisof brines, hydrochloric acid, and sulphates, electroplating,electrowinning, electrolytic production of metal powders, electrolyticcleaning, electrolytic pickling, and the electrochemical generation ofelectric power, involve the use of non-consumable anodes. Previously,graphite anodes have been used in many of these processes, especially insuch processes as the electrolysis of brines and the electrolysis ofhydrochloric acid. More recently, electrodes have been developed forsuch processes utilizing a suitable electroconductive base or substrateand an electrocatalytic coating thereon. Typically such electrocatalyticcoatings have been the platinum group metals; e.g., platinum, osmium,iridium, ruthenium, palladium and rhodium, as well as their oxides.

SUMMARY OF INVENTION It has now been found that a particularlysatisfactory electrode for the conduct of electrochemical reactions maybe provided by the use of a pyrochlore surface on a suitableelectroconductive substrate or base member. Pyrochlores are oxides of ahigh electrical conductivity, low chlorine overvoltage, and highchemical resistance.

DETAILED DESCRIPTION OF THE INVENTION According to this invention anelectrode is provided having a pyrochlore surface on anelectroconductive substrate. Pyrochlores are metal oxycompounds havingthe stoichiometric formula where M is an ion of yttrium, thallium,indium, lead, scandium, silver, cadmium or the rare earth metals,lanthanum, cerium, praseodymium, neodymium, Samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,and lutetium. M is an ion of platinum, palladium, titanium, tin,chromium, rhodium, iridium, antimony, lead, germanium, tungsten,selenium and gold. x is a number in the range of to 2, and y is a numberin the range of 0 to 2.

In quaternary metal ion and tertiary metal ion pyrochlores, mostcommonly M will be lead and M' will be iridium. In pyrochlorescontaining the ions of only two metals x will be 0, y will be either 0or 2, and the pyrochlore will contain bismuth and either ruthenium,rhodium or iridium.

Pyrochlores provide a particularly satisfactory electrode materialbecause they combine a high electrical conductivity comparable to thatof the metals, with a high resistance to chemical attack comparable tothat of the refractory metal oxides. Moreover, pyrochlores provide thehigh surface area normally associated with the oxides of the platinumgroup metals and the low chlorine overvoltages normally associated withthe platinum group metals themselves.

The bulk electrical conductivity of compressed powders of BigRuzoq is onthe order of about 0.5 x10 to about 2 10 (ohm centimeters)- Moreover,the pyrochlores, such as bismuth-ruthenium oxide bismuth-rhodium oxideand bismuth-iridium oxide, are resistant to attack by aqua regia at C.and by nascent chlorine. Furthermore, when applied as a thin coat on atitanium base, Bi Ru oq provides an electrode having a chlorineovervoltage of about 0.06 volt at a current density of 200 amperes persquare foot.

Pyrochlores are a family of oxides having a cubic crystal structure withan Fd3m crystal habit, a 227 crystal number, and a Schoenfiies number of0 The crystallography of the pyrochlores is particularly described in J.Montmory and -F. Dertaut, Pyrochlor-Related Structures, Compt. Rend.,252, 4171 (196 1); I. Longo and I. Goodenough, Preparation andProperties of Oxygen-Deficient Pyrochlores, Mater, Res. Bulletin, 4 (3)191 (1969).

While an electrode that is a bulk pyrochlore body is within thecontemplated scope of this invention, such electrodes will not normallybe utilized for reasons of economy. Preferably, the pyrochloreelectrodes of this invention will be in the form of an electrode havinga pyrochlore surface on a suitable electroconductive substrate. Thepyrochlore surface may be as thin as eight angstroms. But, as apractical matter, the surface will be at least 60 microinches thick and,preferably, in excess of to 200 microinches thick. The pyrochloresurface need not, however, be greater than about 250 to 300 microinchesin thickness. While a pyrochlore surface greater than 300 microinches inthickness (for example, as thick as 800 microinches or more) may be usedwithout deleterious effect, no additional advantages are obtainedthereby. Most frequently the pyrochlore surface will be from about 60 toabout 300 microinches thick. According to one exemplification of thisinvention, the pyrochlore will be in direct contact with the base memberand the electrolyte.

Additionally, the pyrochlores may be used to provide anelectroconductive layer on the electroconductive base or substrate withthe pyrochlore surface having a further exterior coating of a suitablecatalytic or electrocatalytic material. For example, the exteriorsurface may be an electrocatalyst having a low chlorine overvoltage withthe pyrochlore providing a corrosion-resistant electroconductive layerbetween the exterior surface and the substrate, protecting the substratefrom corrosion. Alternatively, the exterior surface may be a surfacecatalyst having a high porosity to allow the flow of electrolyte andcurrent through the surface catalyst containing layer to the pyrochloresurface and then back through the surface catalyst containing layer tothe bulk of the electrolyte. The external surface, when present, shouldhave a porosity of from about 0.50 to about 0.85.

Suitable surface catalytic materials include for example, bimetalspinels, such as cobalt aluminate (CoAl O nickel aluminate (NiAl O ironaluminate (FeAl- O or similar bimetal spinels. The catalytic exteriorsurface may also be provided by perovskites or perovskite bronzes, suchas the tungsten bronzes, e.g., sodium tungstate, potassium tungstatc,and the vanadium bronzes, e.g., sodium vanadate, potassium vanadate, andthe like. Alternatively, the catalytic exterior surface may contain adelafossite, such as PtCoO PdCo- PdRhO PdCrO and the like.

Alternatively, other electrocatalytic materials, such as ruthenates,ruthenites, rhodates, rhodites, and the like may be used to provide aporous exerior surface on the pyrochlore coating.

According to the exemplification of this invention where a porous,exterior surface is provided atop the pyrochlore, an electrode isprovided having a layer of from about 60 to about 300 microinches thickof BigRllzOq on a titanium substrate, with a. coating on the BizRugoq ofCoAl O the coating having a thickness of from about 100 to about 800microinches and a porosity of from about 0.50 to about 0.80.

According to still another exemplification of this invention, thepyrochlore surface may have interposed between it and the electrolyte aporous layer of a substantially non-reactive material, such as titaniumdioxide, vanadium oxide, tantalum oxide, tungsten oxide, niobium oxide,hafnium oxide, zirconium oxide, and the like, or silicon dioxide. Thisadditional exterior coating serves to provide further mechanicaldurability to the pyrochlore surface. Additionally, these porousexterior coatings pro. vide added surface area for surface-catalyzedreactions of the electrode products, or of the electrode reagents.Preferably, the porous exterior coatings are formed in situ as will bedescribed hereinafter. According to this exemplification an electrode isprepared having a 60 microinch to 300 microinch thick layer of Bi Rh O,on a titanium base, with a porous, external layer of TiO;; from 100microinches to 800 microinches thick.

According to still another exemplification of this invention, variousmaterials may be admixed with the pyrochlore material. For example,conductive materials, such as delafossites, perovskites, perovskitebronzes, platinum group metals, oxides of platinum group metals, andmixed .carbides, nitrides, and borides resistant to the electrolyte maybe admixed with the pyrochlore.

These materials may be present to provide additional conductivity orreactivity. Alternatively, these materials may be present to provideadditional catalytic surface area or reduced chlorine overvoltage.According to this exemplification, for example, an electrode may beprepared having a surface from 60 to 300 microinches thick coiitainingBi Ru O, particles admixed with RuO partic es.

Other materials, such as the oxides of titanium, tantalum, niobium,hafnium, tungsten, aluminum, vanadium and other film-forming metals, aswell as silicon, may be admixed with the pyrochlore to provideadditional durability to the surface. Any oxide which may be formed insitu during the preparation of the surface, is substantiallynon-reactive with the electrolyte and has a crystal structure compatiblewith the pyrochlore structure which may be used to bind the pyrochloreparticles to the substrate. According to this exemplification anelectrode may be prepared having from about twenty to about ninetyweight percent TiO admixed with Bi Rh O in a surface from about 60 toabout 300 microinches thick.

As described hereinabove, while the pyrochlore-coated electrodes of thisinvention may include an electrode that is a solid bulk pyrochlore mass,such electrodes will not normally be utilized for reasons of economy.Preferably, the pyrochlore electrodes of this invention will be in theform of an electrode having a pyrochlore surface on a suit ble electrocoductive substrat By suitable electroconductive substrates or basememhers is meant a substrate having an electrical resistivity withineconomic limits for its intended use, e.g., 10 (ohmcentimeters) orhigher, and being substantially non reactive with the electrolyte andthe products of electrolysis. For example, in the electrolysis ofbrines, a suitable electroconductive substrate would be one that issubstantially non-reactive with sodium hydroxide, sodium chloride, orhydrochloric acid solutions and not attacked by nascent chlorine.

The preferred electroconductive substrate or base materials are thevalve metals. The valve metals are those metals which form an oxide filmunder anodic conditions. The valve metals include titanium, tantalum,niobium, hafnium, tungsten, aluminum, zirconium, vanadium, and alloysthereof. For reasons of cost and availability, titanium and titaniumalloys are preferred as the substrate for the electrodes of thisinvention. Where the electrodes of this invention are intended for usein chlor-alkali cells, the valve metal substrates are substantiallyimpervious to the electrolyte. That is, the valve metal base ischaracterized by the substantial absence of pores and interstices suchthat the interior of the valve metal base is not wet by the electrolyte.However, the electrodes of this inven tion may be in the form of arraysof rods and bars, or in the form of mesh or perforate or foraminoussheets, thereby allowing for passage of electrolyte and gases around theelectrode structure. Such electrodes, while themselves macroscopicallyelectrolyte permeable to the bulk flow of electrolyte have members thatare microscopically impermeable to the flow of electrolyte.

Alternatively, other materials such as graphite or carbon may be used asthe electroconductive substrate or base material without deleteriouseffects. A laminate of a valve metal and a less expensive metal such asiron or steel may be used with a pyrochlore coating on the valve metal.For example, an electrode may be provided having a inch thick titaniumsheet bonded to a steel plate, with a to 300 microinch thick pyrochloresurface on the titanium sheet.

Alternatively, titanium hydride or other electroconductive,anodically-resistant hydrides may be used as the electroconductivesubstrate or base member of the electrode of this invention. The hydridemay be present as the sole base member, or it may be in the form of aplate of the hydride on another material. For example, the hydride maybe present as a hydride surface of the metal used in providing the base,e.g., a titanium base with a titanium hydride layer between the titaniummetal base and the pyrochlore surface.

A layer of an electroconductive material more conduc tive than thepyrochlore and also resistant to the electrolyte may be interposedbetween the pyrochlore and the substrate or base member. Suchintermediate layer may be a platinum group metal such as metallicruthenium, rhodium, palladium, osmium, iridium, or platinum, or alloysthereof. Particularly satisfactory alloys include platinum-palladiumalloys, especially those having from 3 to about 15 percent platinumbased on the total weight of the platinum and palladium; andplatinum-iridium alloys, especially those having from about 2 to about50 weight percent iridium based on the total weight of the platinum andiridium. Alternatively, the intermediate layer may be an oxide of aplatinum group metal such as ruthenium oxide, rhodium oxide, palladiumoxide, osmium oxide, iridium oxide, or platinum oxide, or mixturesthereof. Such mixtures may include mixtures of platinum oxide andpalladium oxide having from about 3 to about 15 weight percent platinumoxide based on the total weight of the palladium oxide and platinumoxide, or platinum oxide and iridium oxide containing from about 2 toabout 50 weight percent iridium oxide based on the total weight of theplatinum oxide and the iridium oxide. Such an intermediate layer mayalso contain mixtures of one platinum group metal and the oxide thereofor mixtures of one platinum group metal and the oxide of anotherplatinum group metal. Additionally, oxides of other metals such astitanium, zirconium, hafnium, vanadium, niobium, tantalum, tungsten, andaluminum may be present in the intermediate coating with the platinumgroup metals or the oxides thereof. The intermediate layer may alsocontain electrically conductive, corrosion resistant, oxygen containingcompounds of the platinum group metals. Such compounds include thedelafossites, such as PtCoO PdCoO PdCrO PdRhO PdRuO and PdPbOg.

The intermediate layer, when present, is normally from about 20 to about120 microinches thick, and preferably from about 60 to about 120microinches thick. It may be thinner, for example, as thin as 5microinches, if applied uniformly so as to provide a pore-free coating.It may also be thicker, but without any significant effect. Thus,according to this exemplification, an electrode may be provided having aBi Ru o pyrochlore layer 60 to 300 microinches thick on a titanium basewith an intermediate layer of PtCoO delafossite, from 60 to 120microinches thick, between and in electrical contact with the titaniumand the pyrochlore.

Alternatively, where an electrode is provided having a pyrochloresurface on a. substrate, where the substrate material reacts with thepyrochlore to form an electrically insulating barrier after extendedperiods of electrolysis at high current density, there may be interposedLetween the substrate and the pyrochlore a layer of less-reactivematerial that is more resistant to the electrolyte than is the substrateor base member. By less-reactive material is meant a material that doesnot form an electricallyinsulated barrier or aid in the formation ofsuch a barrier when interposed between the pyrochlore and substrate.This less-reactive material may be a precious metal or oxide thereof asdescribed hereinabove. The intermediate layer, when present, is normallyfrom about 20 to about 120 microinches thick, and preferably from about60 to about 120 microinches thick. It may be thinner, for example, asthin as 5 microinches, if applied uniformly so as to provide a pore-freelayer. It may also be thicker, but without any significant effect.

The pyrochlore surface electrodes of this invention are useful in anyelectrochemical process where a non-consumable electrode is used. Bynon-consumable is meant an electrode that is not dissolved by theelectrolyte and redeposited on the opposite electrode. For example, theelectrodes of this invention may be used in the electrolysis of brines,sulphates, hydrochloric acid, phosphates, and the like. Alternatively,the electrodes of this invention are useful in those electrolyticprocesses where cations are deposited on a cathode; for example, anelectrotrowinning, electrorefining, electroplating, electrophoresis,electrolytic cleaning, electrolytic pickling. Copper, nickel, iron,manganese, brass, bronze, cadmium, gold, indium, silver, tin, zinc,cobalt, chromium, and the like may be electroplated in suitablesolutions onto cathodes. Alternatively, metal powders may be prepared bydepositing cations out of solution onto a suitable cathode using theelectrodes of this invention. The electrodes of this invention may beused for electrolytic cleaning using aqueous solutions of sodiumphosphate, sodium carbonate, and the like. Additionally, electrolyticpickling of suitable materials rendered cathodic with respect to theanode of this invention may be carried out using the anode of thisinvention. The anode of this invention may also be used for electrolyticoxidation of organic compounds. For example, the electrolytic oxidationof propylene to propylene oxide or propylene glycol may be carried outusing the electrodes of this invention. Metal structures such as shipshulls may be cathodically protected using the anodes of this invention.

In each use of the electrodes of this invention enumlerated above, thecell comprises an electrode pair having an anode and a cathode, at leastone member of the electrode pair being the pyrochlore-surfaced electrodeherein contemplated, and means to establish an external voltage orelectromotive force between the anode and the cathode whereby the anodeis positively charged with respect to the cathode and an electricalcurrent is caused to pass from one member of said electrode pair to theother member.

Additionally, the electrodes of this invention may be used in fuelcells. When used in fuel cells the pyrochlore surface of the electrodesmay be penmeableto the flow of electrolyte. Such electrolytepermeability may be provided by an electroconductive substrate havingpyrochlore deposited therein, or by a dispersion of pyrochlore particlesin a suitable, inert media, or by other methods known in the art. Fuelcells using the electrode of this invention include apyrochlore-containing electrode, an electrode of opposite polarityspaced from the pyrochlorecontaining electrode, apparatus for feedingelectrolyte into the space between the electrodes in order to internallygenerate electromotive force between said electrodes, and apparatus forrecovering the electrical energy generated within the fuel cell.

The pyrochlores useful in providing the electrode surface of the presentinvention may be synthesized and include those synthesized by themethods described in US. Pat. 3,583,931 to Bouchard; and Solid StateResearch, Lincoln Laboratory Report No. ESD-PR-66-403, pp. 21-22 (1966)by Longo et al.

A preferred method of synthesizing the pyrochlore useful in providingthe electrode of this invention involves heating a finely divided sourceof an oxide of a platinum group metal with a source of bismuth oxide (BiO at a temperature of approximately 600 C. or higher. This reaction maybe carried out either in the presence of oxygen or in the absence ofoxygen. Most commonly the oxide of the platinum group metal is rutheniumoxide, rhodium oxide, or iridium oxide. The oxides of bismuth andruthenium, rhodium, or iridium are typically present in an atomic ratioof one atom of bismuth as Bi O to the one atom of the platinum groupoxide. The oxides are ground together and the mixed powders are fired inan evacuated, sealed tube. The reaction is carried out at a temperaturefrom about 600 C. to about 1100 C. and preferably from about 750 C. toabout 1000 C. and most commonly from about 750 C. to about 850 C. Timeof reaction is from about 1 hour to about 72 hours and most commonlyfrom about 16 hours to about 48 hours.

The particular times and temperatures necessary for the synthesis of thepyrochlores useful in providing the electrode coatings contemplatedherein depend upon the particle size of the precursors. For example, asatisfactory pyrochlore is obtained if minus 325 mesh powders of Bi O;and Rh O mixed and compressed into one-quarter inch pellets, are heatedto 800 C. for from about 36 to about 42 hours. The preformed pyrochloremay then be applied to the substrate.

Alternatively, pyrochlore may be formed in situ on the substrate itself.This is carried out by firing bismuth (HI) oxide powders and ruthenium,rhodium, or iridium powders in the proper proportions on the substrateitself. According to this exemplification bismuth oxide (Bi O which iswater-soluble, may be put into solution with a nitrate or chloride ofthe platinum group metal. This solution may then be applied directly tothe substrate. The substrate, with the solution of Bi O and the salt ofthe platinum group metal, is then heated to from about 300 to about 500C. to drive ofi the volatiles and convert the salt to the oxide. Thenthe substrate is heated to from about 500 C. to about 600 C. to convertthe oxides to the pyrochlore.

When a titanium member is utilized as the base or substrate member, thetitanium member is prepared for use as an electrode substrate or basemember by degreasing an etching prior to deposition of the pyrochloresurface. Degreasing may be carried out by any of the methods well knownin the art such as by the use of detergent, abrasives, or organicdegreasing agents. Thereafter, the greased titanium base or substratemember is etched in hydrochloric acid or the like. The etching serves toremove the naturally occurring oxide film and substitutes therefore ahydride film.

The pyrochlore surface may be applied to the etched titanium substratein a number of ways. For example, when the pyrochlore surface isprepared in situ, a liquid composition containing bismuth oxide andruthenium chloride in a volatile solvent or a thermally-decomposableorganic solvent is applied to the titanium base. The titanium basecontaining the liquid composition is then heated to a temperature offrom about 300 C. to about 500 C. for from about 10 minutes to about 1hour after the application of each coat, and then to from about 500 C.to about 500 C. to drive off the volatiles and convert Alternatively,when the pyrochlore is preformed, a slurry of the pyrochlore may beprepared in a solvent such as ethanol, butanol, benzyl alcohol, phenol,benzene, cumene, or the like. The slurry may be brushed onto the surfaceof the titanium followed by heating to decompose or volatilize thesolvent after each coat. The slurry may, additionally, contain acompound of silicon, titanium, zirconium, hafnium, vanadium, niobium,tantalum, molybdenum, tungsten, or other material capable of forming anoxide in situ during the process of volatilizing or decomposing thesolvent. Most frequently such compounds will be a chloride such astitanium trichloride, TiCl or a nitrate such as zirconium oxynitrateZrO(NO Alternatively, a slurry of the pyrochlore, an organic solventsuch as phenol, butanol, or benzyl alcohol, ethanol, benzene, cumene,polyene, or the like, and either a silica compound or a metal compoundthat is soluble or dispersable in the organic solvent, e.g., a resinate,may be prepared. This slurry may be brushed onto the titanium providing,for example, from about 4 to about 8 coats of the slurry, with heatingafter each coat to decompose or volatilize the organic constituent. Afinal heating to about 500 C. or 600 C. for from about 10 minutes to onehour or longer may follow the heating after each coat. Alternatively,methods such as compression bonding or cathodic electrophoresis may beused to apply the coating of pyrochlore.

While the above-described methods of coating the titanium substrate withpyrochlore have described the use of a titanium, zirconium, or siliconcompound to provide TiO ZrO or Si in the surface, this is not necessaryfor the function of the electrode. However, the titanium dioxide,silicon dioxide, or zirconium dioxide does provide additional strengthand durability to the pyrochlore surface. Alternatively, oxides ofhafnium, vanadium, niobium, tantalum, molybdenum, tungsten, or the likemay be formed in situ during the formation of the surface to provide adurable electrode surface.

The resulting electrode, prepared as described above, may be utilized asan electrode for the conduct of electrochemical reactions ashereinbefore described. The following examples are illustrative.

EXAMPLE I An electrode was prepared having a Bi Ru O pyrochlore surfaceon a titanium substrate. A titanium coupon by by inch was washed withCornet (TM), a household cleanser containing abrasives and cleansers,rinsed in distilled water, and dipped in a 1 weight percent hydrofluoricacid solution for 1 minute. Thereafter, the coupon was inserted in 12normal hydrochloric acid at 27 C. for 23 hours.

A solution was prepared containing 0.1860 gram of Bi 0 0.2108 gram ofRuC1 .3H O, 0.090 gram of ZrO(NO .nH O, 2.0 grams of 4 normal I-INOcontaining 1 weight percent of GAF IGEPAL 887 nonyl phenoxypolyethyleneoxy ethanol. Six coats of this solution were applied to thetitanium coupon. After each of the first and second coats the coupon washeated to 300 C. for 10 minutes. After both the third and fourth coatsthe coupon was heated to 350 C. for 10 minutes. After both the fifth andsixth coats the coupon was heated at 400 C. for 15 minutes.

Thereafter a slurry was prepared containing 0.50 gram ofpalladium-cobalt delafossite prepared as described in commonly-assigned,copending application Ser. No. 222,501, filed Feb. 1, 1972, 0.50 gram ofa 4.5 weight percent titanium (calculated as the metal) solution'oftitanium tetrachloride in butanol, 0.375 gram of butanol containing 1weight percent IGEPAL CO-8 87, and 0.30 gram of phenol. Two coats ofthis slurry were applied on the pyrochlore surface of the coupon. Thecoupon was heated at 400 C. for 10 minutes. A coat of 2.25 weightpercent titanium as titanium tetrachloride in butanol was then appliedand heated to 400 C. for 10 minutes. Two additional coats of thepalladium-cobalt delafossite solution described above were applied tothe titanium coupon heating to 400 C. for 10 minutes. A second coat of2.25 weight percent titanium as titanium tetrachloride in butanol wasapplied to the coupon and heated to 400 C. for 10 minutes.

An additional coat of the palladium-cobalt delafossite slurry preparedas described above was applied to the coupon and heated to 400 C. for 10minutes. A final coat of 2.25 weight percent titanium as titaniumtetrachloride in butanol solution was then applied to the coupon andheated to 400 C. for 10 minutes. The coupon was then further heated to600 C. for 60 minutes.

The electrode having a palladium-cobalt delafossite surface with anintermediate pyrochlore layer of titanium substrate was tested as theanode in a beaker chlorate cell. The electrolyte was a solution ofsodium chloride containing 310 grams per liter of sodium chloride.Electrolysis was commenced and chlorine was seen to be evolved. Thechlorine overvoltage was 0.04 volt at 200 amperes per square foot and0.07 volt at 500 amperes per square foot.

The electrode was then vigorously brushed with a wire brush until theX-ray diffraction pattern showed the presence of both the delafossiteand a micro-crystalline pyrochlore on the surface of the electrode. Theelectrode was then tested as an anode. Chlorine was seen to be evolvedfrom the anode, Which had a chlorine overvoltage of 0.04 volt at 200amperes per square foot and 0.05 volt at 500 amperes per square foot.

EXAMPLE II An electrode was prepared having a Bi Ru O intermediatesurface on a titanium base with a palladiumcobalt delafossite exteriorsurface thereon.

A solution was prepared containing 0.1860 gram of Bi O 0.2108 gram ofRuCl .3H O, 2.0 grams of 4 N HNO and 1 weight percent JGEPAL CO-887. Asecond solution was prepared containing 2.25 weight percent titanium astitanium tetrachloride in butanol.

One coat of the bismuth oxide-ruthenium trichloride solution was appliedto a titanium coupon that had been etched and cleaned as described inExample I hereinabove. The coupon, with the one coat solution thereon,was heated to 300 C. for 10 minutes. A second coat of the bismuthoxide-ruthenium trichloride solution was brushed on the coupon andthereafter, without subsequent heating, a coat of the titaniumtetrachloride solution was applied. The coupon was then heated to 300 C.for 10 minutes. Thereafter two coats of the bismuth oxiderutheniumtrichloride solution were applied to the titanium coupon. The coupon washeated to 350 C. for 15 minutes after each of the coats. A subsequentcoat of the titanium tetrachloride solution was applied atop the bismuthoxide-ruthenium tetrachloride coat. Thereafter the coupon was againheated to 350 C. for 15 minutes.

Two additional coats of bismuth oxide-ruthenium trichloride solutionwere applied to the coupon. After each of the coats the coupon washeated to 400 C. for 15 minutes. Thereafter an additional coat of thetitanium tetrachloride solution was applied and the coupon was heated to400 C. for 15 minutes.

A palladium-cobalt delafossite slurry was prepared and applied atop thebismuth-ruthenium pyrochlore surface as described in Example Ihereinabove, was heated to 575 C. for 60 minutes. Thereafter theelectrode having an exterior palladium-cobalt delafossite surface andintermediate bismuth-ruthenium pyrochlore surface layer on a titaniumsubstrate was tested as the anode in a beaker chlorate cell as describedin Example I hereinabove. Chlorine was observed to be evolved. Thechlorine overvoltage of the anode was found to be 0.05 volt at 200amperes per square foot and 0.08 volt at 500 amperes per square foot.

EXAMPLE III An electrode was prepared having a bismuth-rhodium oxidepyrochlore surface on a titanium substrate. Rhodium oxide was preparedby heating Engelhard Rh-110 rhodium oxide hydrate in air at 800 C. for18 hours. The dehydrated Rh O Was mixed with B and A Reagent Grade Bi Oto yield a mixture of 0.9030 gram of RH O and 1.65 80 grams of Bi O Thismixed powder was ground and packed into an Alundum boat. The boatcontaining the Rh O and the Bi O was heated to 775 C. per 20 hours.Thereafter the resulting product was reground and repacked into anAlundum boat and heated for 36 hours at a temperature between 796 C. and805 C. The resulting crystalline product was determined by X-raydiffraction to have the pyrochlore structure and to contain noobservable amount of either RH O or Bi O The X-ray diffraction patternof the powder sample is shown in Table 1.

A slurry was prepared containing 0.20 gram of 0.30 gram of titanium(calculated as the weight of the metal) in a 4.2 weight solution ofEngelhard Titanium Resinate, 0.15 gram of toluene, and 0.05 gram ofphenol. Five coats of this slurry were brushed onto a titanium couponthat had been cleaned and etched as described in Example I hereinabove.After each coat the coupon was heated at the rate of 50 C. per 5 minutesto 400 C. and maintained thereat for minutes.

Thereafter a solution containing 2.25 weight percent of titanium(calculated as the metal) in a solution of titanium tetrachloride inbutanol was prepared. One coat electrode, having a bismuth-rhodiumpyrochlore sursurface and the coupon was heated to 120 C. for minutes.Thereafter the coupon was heated to 500 C.

and maintained thereat for 20 minutes. The resulting electrode, having abismuth-rhodium pyrochlore surface on a titanium substrate, was testedas the anode in a beaker chlorate cell as described in Example Ihereinabove. At a current density of 200 amperes per square foot theelectrode had a chlorine overvoltage of 0.10 volt and chlorine was seento be evolved.

EXAMPLE IV An electrode was prepared having a bismuth-rutheniumpyrochlore surface on a titanium substrate. Bismuthruthenium oxidepyrochlore was prepared by mixing 1.8718 grams of ruthenium dioxide and3.262 grams of bismuth oxide powders together. These were groundthoroughly and placed in an Alundum boat. The Alundum boat containingthe mixed powders was heated to 790 C. for 16 hours. The resultingproduct was removed from the Alundum boat, reground, repacked into theAlundum boat, and heated at 780 C. for 19 hours. The resulting powderhad an X-ray diffraction pattern showing sharp peaks identified with thepyrochlore structure and also small amount of ruthenium dioxide.

To the above product was added 0.16 gram of bismuth oxide. The powderwas placed in an Alundum crucible and heated at 800 C. for 27 hours. Thebismuth-ruthenium pyrochlore obtained thereby was then leached withmilliliters of two normal HCl and 100 milliliters of one normal HCl,washed with water and with acetone.

A slurry was prepared containing 0.20 gram of bismuth-rutheniumpyrochlore, 0.30 gram of Engelhard Titanium Resinate (containing 4.2percent titanium calculated as the metal), 0.15 gram of toluene, and0.05 gram of phenol. Five coats of this solution were applied to atitanium coupon that had been cleaned and etched as described in ExampleI hereinabove. After each of the five coats of slurry were applied, thecoupon was heated at the rate of 50 C. per 5 minutes to 400 C. andmaintained thereat for 10 minutes. Thereafter, one coat of a solution oftitanium tetrachloride in butanol containing 2.25 percent titaniumcalculated as the metal was applied and heated first to C. for 20minutes and thereafter to 500 C. for 20 minutes.

The resulting electrode having a bismuth-ruthenium oxide pyrochloresurface on a titanium substrate was tested as the anode in a beakerchlorate cell. Chlorine was observed to be evolved from the anode. Theresulting electrode had a chlorine overvoltage of 0.06 volt at 200amperes per square foot and 0.12 volt at 500 amperes per square foot.

EXAMPLE V The balance of the pyrochlore obtained in Example IV above wasthen reground, 0.25 gram of bismuth oxide was added to it, and it wasplaced in an Alundum boat and heated to 790 C. for 20 hours. Theresulting product was removed from the Alundum boat, leached five timesin two normal hydrochloric acid, washed with water and then withacetone.

The X-ray diffraction pattern showed the sharp peaks characteristic ofbismuth-ruthenium pyrochlore and a second material having the Bi Ru Ostoichiometry. The X-ray diffraction pattern of the powder sample isshown in Table 2.

A slurry was prepared containing 0.20 gram of the BizRuzoq so prepared,0.30 gram of a solution of titanium tetrachloride in butanol (containing4.5 weight percent titanium calculated as the metal), 0.10 gram ofbutanol, and 0.15 gram of phenol. Five coats of this slurry were appliedto a titanium coupon that had been degreased and etched as described inExample I hereinabove. After each of the first four coats the coupon washeated to 120 C. for 20 minutes, and then to 350 C. for 15 minutes.

11 TABLE 2 d value: (I/I,,) 100 6.55 5

After the last coat the coupon was heated to 500 C. for minutes.

The resulting electrode, having a BizRuzoq surface on a titanium base,was tested as the anode in a laboratory chlorine cell. Chlorine was seento be evolved, and the chlorine overvoltage was 0.09 volt at 200 amperesper square foot and 0.19 volt at 500 amperes per square foot.

It is to be understood that although the invention has been describedwith specific reference to specific details of particular embodimentsthereof, it is not to be so limited since changes and alterationstherein may be made which are within the full intended scope of thisinvention as defined by the appended claims.

What is claimed is:

1. An electrode having a valve metal substrate and an electroconductivesurface thereon comprising:

a pyrochlore chosen from he group consisting of" Bi Ru O and BigRhgoq;and an oxygen-containing compound chosen from the group consisting ofperovskites, delafossites, and oxides of titanium, tantalum, zirconium,columbium, hafnium, tungsten, aluminum, vanadium, silicon, ruthenium,rhodium, palladium, osmium, iridium, and platinum. 2. The electrode ofclaim 1 wherein the electroconductive surface contains from about ten toabout eighty weight percent pyrochlore.

3. The electrode of claim 2 wherein the oxygen-containing compound istitanium dioxide and the electroconductive surface contains from abouttwenty to about ninety weight percent titanium dioxide.

4. In an electrolytic cell having an anode, a cathode, and externalmeans for establishing an electromotive force between said anode andsaid cathode whereby to cause an electrical current to pass from saidanode to said cathode, the improvement wherein said anode comprises avalve metal substrate having an electroconductive surface thereoncomprising:

a pyrochlore chosen from the group consisting of Bi Ru O and Bi Rh O andan oxygen-containing compound chosen from the group consisting ofperovskites, delafossites, and oxides of titanium, tantalum, zirconium,columbium, hafnium, tungsten, aluminum, vanadium, silicon, ruthenium,rhodium, palladium, osmium, iridium, and platinum. 5. The electrolyticcell of claim 4 wherein the electroconductive surface of the anodecontains from about ten to about eightly weight percent pyrochlore.

6. The electrolytic cell of claim 5 wherein the oxygencontainingcompound is titanium dioxide and the electroconductive surface containsfrom about twenty to about ninety percent titanium dioxide.

7. In a method of electrolysis wherein brine is fed to an electrolyticcell, an electrical current is caused to pass from an anode to acathode, and chlorine is generated at the anode, the improvement whereinsaid anode comprises a valve metal substrate having an electroconductivesurface thereon comprising:

a pyrochlore chosen from the group consisting of BizRuzoq and BlzRhgoq;and

an oxygen-containing compound chosen from the group consisting ofperovskites, delafossites, and oxides of titanium, tantalum, zirconium,columbium, hafnium, tungsten, aluminum, vanadium, silicon, ruthenium,rhodium, palladium, osmium, iridium, and platinum.

8. The method of electrolysis of claim 7 wherein the electroconductivesurface of the anode contains from about ten to about eightly Weightpercent pyrochlore.

9. The method of electrolysis of claim 8 wherein the oxygen-containingcompound in said electroconductive surface of the anode is titaniumdioxide and the electroconductive surface contains from about twenty toabout ninety weight percent titanium dioxide.

References Cited UNITED STATES PATENTS 3,691,052 9/1972 Langley 204-290F 3,711,397 1/1973 Martinsons 204290 F 3,718,551 2/ 1973 Martinsons204290 F FREDERICK C. EDMUNDSON, Primary Examiner US. Cl. X.R. 204290 FPa an: No. 3 801 490 Dated April 2, 1974 Enventorfla) Cletus Welsh i;certifie'i error appears in the above-identified patent that saiclLetters Patent are hereby corrected as shown below:

line 4, it she state --assignor to Nora International City,

In Gal-12m ll, iifle Q3 he should be -'E:he--.

Signed end seaiei this 10th day of September 1974 C MARSHALL DANNCommissioner of Patents USCQMM'DC 5Q376-P6D U a GOVEREHENT PRINTINGOFFICE: 196! 0-366-53l,

